The preppy watch must be classic, classy and conservative. Never tacky, never bulky

So glad you asked. I was about to do a piece on watches!
I wear two watches: my J. Crew Timex and the Kate Spade watch. I guess it depends what type of “preppy” you are. New prep (Lacoste, Rugby, etc) or Old Prep (Brooks Brothers, Ralph Lauren, etc).
If money is not an issue, you could always go with the classic Hermes Kelly Watch. It’s something I’ve had my eye on for a while.
Good luck